1. Field of the Invention
The present invention relates to an improvement on a chip resistor.
2. Description of the Prior Art
Chip resistors that have conventionally been in wide use are produced by forming a resistive layer on the surface of an insulating substrate, providing electrodes at both ends of the resistive layer, and forming one or more protective films on the surface of the resistive layer. FIG. 4 shows a vertical section of such a chip resistor. This figure shows a chip resistor having three protective films formed on the surface of its resistive layer, with numeral 1 representing an insulating substrate made of, for example, ceramics, numeral 2 representing a resistive layer formed on the surface of the substrate 1, numeral 3 representing electrodes provided at both ends of the resistive layer 2, numeral 4 representing a resistive-layer protection film, numeral 5xe2x80x2 representing an intermediate protection film, and numeral 6xe2x80x2 representing a surface protection film. Each protection film is made essentially of glass paste. The electrodes 3 have their surfaces metal-plated.
The protection films are formed as follows. First, the material for the resistive-layer protection film 4 is applied to the surface of the resistive layer 2, and is then subjected to drying and calcination. At this time, the resistive-layer protection film 4 serves to reduce the variation of (i.e. stabilize) the resistance of the resistive layer 2 under calcination. Thereafter, the resistive layer 2 is trimmed, for example, with a laser beam for the adjustment of its resistance. Subsequently, the intermediate protection film 5xe2x80x2 is applied to the surface of the resistive-layer protection film 4 and is then subjected to drying. Subsequently, the surface protection film 6xe2x80x2 is applied to the surface of the intermediate protection film 5xe2x80x2 and is then subjected to drying. Lastly, the surface protection film 6xe2x80x2 is subjected to calcination. It is also possible to subject the intermediate protection film 5xe2x80x2 to calcination before the application, drying, and calcination of the surface protection film 6xe2x80x2.
In general, the resistive-layer protection film 4 is provided, as described above, for the purpose of reducing the variation of the resistance of the resistive layer 2 under calcination; the intermediate protection film 5xe2x80x2 is provided for the purpose of filling trimming grooves that are left after the above-mentioned trimming; the surface protection film 6xe2x80x2 is provided for the purpose of protecting the resistor against mechanical force that may be applied from outside. Thus, in a conventional chip resistor, these protection films, to serve their respective intended purposes, need to be made of materials having different properties in terms of their softening point, Vickers hardness, thermal expansion coefficient, and others. This leads to the following inconveniences.
For one thing, when a chip resistor, in the manufacturing process, receives mechanical force from outside, the chip resistor may develop, as shown in FIG. 5, a crack that penetrates completely through the surface protection film 6xe2x80x2 and the intermediate protection film 5xe2x80x2 but only halfway into the resistive-layer protection film 4. In actual use, a chip resistor with such a crack, when heat is applied thereto during soldering, often ends in the crack reaching the resistive layer 2 and thus the chip resistor having a resistance different from the intended resistance. In this case, exactly because each protection film is made of a different material, the crack tends to take a non-linear path and thus remain inside, without coming to the surface.
Alternatively, in cases where the intermediate protection film 5xe2x80x2 is made of a mechanically weak material, the chip resistor may develop, as shown in FIG. 6, multiple cracks in the intermediate protection film 5xe2x80x2, and in addition the differences in the thermal expansion coefficient between the protection films cause stress to be present at all times between those films. This makes the chip resistor susceptible to a thermal shock such as is caused by soldering.
Moreover, the difference in the softening point between the intermediate protection film 5xe2x80x2 and the surface protection film 6xe2x80x2 makes it difficult to determine the appropriate calcination temperature. This leads to instability of the resistance of the resistive layer 2 under calcination, or causes, in the protection films, defects such as pinholes that do not come to the surface. These faults are difficult to detect in the inspection process, and thus chip resistors having such a fault are in many cases shipped out as non-defective products, with their fault unnoticed.
An object of the present invention is to provide a chip resistor that reveals a crack for permitting easy detection of it in the inspection process, that suffers from minimum variation of the resistance during calcination of a protection film, and that is not prone to defects such as pinholes that do not come to the surface.
To achieve the above object, according to the present invention, in a chip resistor produced by forming a resistive layer on the surface of an insulating substrate, providing electrodes at both ends of the resistive layer, forming a resistive-layer protection film on the surface of the resistive layer, forming an intermediate protection film on the surface of the resistive-layer protection film, and forming a surface protection film on the surface of the intermediate protection film, the resistive-layer protection film, the intermediate protection film, and the surface protection film are all made of an identical material.
Alternatively, in a chip resistor produced by forming a resistive layer on the surface of an insulating substrate, providing electrodes at both ends of the resistive layer, forming a resistive-layer protection film on the surface of the resistive layer, and forming a surface protection film on the surface of the resistive-layer protection film, the resistive-layer protection film and the surface protection film are both made of an identical material.
In these chip resistors, the protection films are made essentially of lead-borosilicate glass of an identical composition. More specifically, the lead-borosilicate glass preferably has the following properties:
Moreover, the protection films are preferably made of glass paste that contains lead-borosilicate glass in the form of particles 2-10 xcexcm across and that contains terpineol or butyl carbitol acetate as solvent.